In a well known and widely utilized process for the manufacture of cellulose ester films, a "dope" is formed by dissolving a cellulose ester in a solvent or mixture of solvents, the dope is cast upon a smooth surface such as a stainless steel belt or a casting wheel with a nickel-plated or chromium-plated surface, part of the solvent is evaporated from the dope in order to form a material with sufficient strength to retain its form, and the film is stripped from the surface upon which it was cast. In this process, the speed at which the casting operation can be successfully conducted is dependent upon how quickly the film can be stripped from the casting surface, which in turn depends upon how fast the film attains sufficient strength to be stripped. If the film is stripped prematurely, a problem of delamination is encountered, i.e., the coating tends to split within its thickness, leaving part of the composition adhered to the casting surface.
It is well known to incorporate a stripping aid in the cellulose ester composition in order to reduce the time required before the coating can be stripped from the casting surface without encountering delamination. The stripping aids function to reduce the tension required to strip the film from the casting surface, and/or to promote the rapid attainment of film strength, and thereby enable the film to be stripped sooner than is feasible without their use. A variety of materials are known to be useful as stripping aids in cellulose ester compositions including, for example, the dialkyl esters of sulfosuccinic acid of U.S. Pat. No. 2,275,716, the fatty acids of U.S. Pat. No. 3,528,833, and the metallic salts of U.S. Pat. No. 3,793,043. However, materials heretofore proposed for this purpose often exhibit a less than adequate degree of improvement in stripping characteristics and/or cause deleterious effects which limit the usefulness of the resulting product. For example, the dialkyl esters of sulfosuccinic acid can accelerate the degradation of cellulose esters, fatty acids can interfere with the application and adhesion of subsequent coatings, and metallic salts can impart an undesirable coloration to the film which interferes with photographic applications.
It is toward the objective of providing new and improved film-forming cellulose ester compositions, containing novel stripping aids which exhibit a highly desirable combination of characteristics that overcome the disadvantages and limitations of the prior art, that the present invention is directed.